Secret phonograph record



.Oct. 19,1926. 1,603,287

J. MILLS VSECRET PHONOGRAPH RECORD Filed Oct. 26. 1922 LOW PASS hven/on John inal sound waves but inverted in order. For

Patented Oct. 19; 1926.

UNITED STATES I 1,603,287 PATENT OFFICE.

JOHN.MII4LS,' OF 'WYOMING, NEW JERSEY, ASSIGNOR T WESTERN ELECTRIC COM- PANY, INCORPORATED, OF NEW YORK, N. Y., A CORPORATION OF NEW YORK.

SECRET PH ONOGRAPH RECORD.

This invention relates to secret phonograph records and to methods of recording and reproducing secret messages.

An object of the invention is to disguise or distort an audible message in recording it in such a manner that the message can not be reproduced intelligibly except by one having" knowledge of the manner in which the message was recorded.

In the system disclosed herein for carrying out the invention, sound waves to: be recorded are inverted by being first converted into electric waves and then caused to modulate a continuous wave having a frequency near the upper frequency of the sound waves. The resultingwaves comprise frequencies representing the original sound frequencies, the frequency of the continuous wave source, the sum of these frequencies and the difference of these frequencies. The latter roup which is known as the lower-side ban comprises frequencies extending through the range of the origexample, if the continuous source has a frequency of 2200 cycles, frequencies of 200 cycles and 2000 cycles whichmay be present in the source of sound will appear in the lowerside band as 2200200 or 2000 cycles and 2200-2000 or 200 cycles respectively.

By electrical means all frequenciesof the modulated wave, except the lower side band are eliminated and the lower side band is.

impressed on a suitable recording device such as a magnetically operated stylus for cutting a record. Obviously the message so recorded would be unintelligible if repr0- duced by means of an ordinary phonograph reproducing means but can be made intelligible by one which will reinvert the frequencies to their original order. For this purposea modulator substantially like the one used for distorting the message is used for restoring it to its original intelligible form.

Other objects of the invention will be apparent from the following detailed description and claims taken in connection with the accompanying drawings in which Fig. 1 represents schematically a recording 4 system which may be employed for carrying out the invention, and Fig. 2 represents a reproducing system.

The recording system shown in Fig. 1

comprisesa suitable transmitter connected Application filed October as, 1922. Serial in. 596,930.

through a potentiometer 16, a low pass filter 18, a balanced modulator M, a vacuum tube oscillator and amplifier 21, a second low pass filter 22' similar to filter 18 and a recording. device 33 which may be in operative relation to a disc34 adapted to have a record cutin its surface. filters are of the type disclosed in Campbell Patent No. 1,227,113, and are so designed as to transmit with practically uniformly negligible attenuation currents of all frequencies less than a fixed cut-off frequency as, for example, 2200 cycles while substantially extinguishing all currents of higher frequencies.

The oscillator 20 comprises the well known three-element electric discharge device having a frequency determining tuned loop consisting of inductance 24 and variable capacity 25 in its plate filament circuit and having its grid filament circuit coupled by an inductance 26 to inductance 24 in a well known manner to enable the device to produce continuous oscillations. The grid filament'ci'rcult of thermionic amplifier 21 is connected directly to that of the oscillator, which accordingly impresses the generated oscillations upon the amplifier, which, in turn, impresses amplified oscillations across the variable resistance 27. The amplifier, oscillator and balanced modulator are each supplied with space current from the common source 28 through choke coils 29 which exclude the oscillatory current, thus preventing direct high frequency coupling through the common space current supply. The various filamentary cathodes of the modulator, oscillator and amplifier may likewise receive heating current from a common source, but individual sources have been shown to simplify the drawing.

The modulator is of the balanced type similar in general to that disclosed in Hartley Patent No. -1,419,562, granted June 13, 1922, and, as is well known, serves to pass only the modulated side bands or sum and difference frequency components of the currents impressed on the input circuit while substantially preventing transmission of unmodulated currents corresponding to the impressed waves.

two side bands produced by modulation with the speech currents will both be within the audible range and the lower side band will The low pass If the oscillator 20 sup-. plies a wave of 2200 cycles frequency, the

occupy substantially the same range as theoriginal speech, but will be unintelligible because it consists entirely of inverse frequencies. For example, the frequencies 800, 1000 and 1200 of the original speech components will have become inverse frequencies 1400, 1200 and 1000 respectively. An increasingpitch tone of speech will become an unintelligible decreasing pitch noise in the inverse frequency band. The filter 22 suppresses the upper side band and permits transmission of only the inverse frequency.

band.

The reproducing system comprises su'bstantially the same general arrangement of apparatus, an electrical re roducer 36' and an amplifier A being su stituted' for the transmitter and filter in Fig. 1, and a re-.,

i 2200 cycles frequency-to produce [two side bands, the lower of which is intelligible speech current. The lower side band is selected and transmitted to the receiver 38 through the low pass filter 37 which suppresses the upper side band. As explained above, the frequencies received from the reproducer and from the oscillator are suppressed in the modulator itself.

The recording and reproducing circuits shown herein are substantially like the transmitting and receiving circuits of the secret telephone system shown in Fig. 2 of the Patent of B. W. Kendall, No. 1,571,010, Jan. 26, 1926.

While only one system for distorting messages has been illustrated, it is to be understood other methods and systems for disguising or distorting speech may be employed, and further, that any desired means of recording the waves may be used.

The term message as used herein is a generic term intended to include any form of lntelligence to be recorded by phonographic methods.

The invention claimed is:

L'The method of recording a message comprising a plurality of frequencies which comprises modulating an audible frequency in accordance with a band of frequencies comprising at least a plurality of the freappear in inverted order, and selecting and .so modified as to be unintelligible, which prises modulating a constant frequency in accordance with said waves, selecting from the modulatedwave. .the lower side band comprising unintelligible inverse frequency waves and recording said side band. 79

3. The method of recording a message comprising a plurality of audible frequencies which comprises producing a wave having a band offrequencies which comprlse at least a portion ofthe frequencies of said message, modulating an audible frequency jjvalve in accordance with the frequencies ef I isaid wave so as to produce a side'band in which thsfrequencies of the message wave so recording said side band.

4. The method which comprises modulating an audible frequency message with a continuous frequency of a value to produce a side band in which the message frequencies appear in inverted order in substantially the same range as the original fre uencies, and selecting and recording sai side band.

5. The method of reproducing a recorded message having a band of frequencies at least a plurality of which have been displaced from their normal positions in the band before recording, which comprises restoring said frequencies to their normal positions and translating the restored frequency waves into sound.

6. The method of reproducing a message which has been recorded with its frequencies comprises converting said distorted message into electric waves, restoring said frequencies to their original form, and converting said electric waves into sound waves.

7. The method of recording a message comprising a sound wave which comprises producing an electric wave having a band of frequencies including at least a portion of the frequencies of said sound wave, modulating said electric wave with an audible frequency, selecting from the modulated wave the lower side band comprising audible but unintelligible inverse frequency waves and recording said side band.

8. The method which comprises modulating an audio frequency continuous wave by a band of frequencies representing sound, to producea lower side band of frequency components each of which has a frequency equal to that ofthe continuous wave minus a corresponding component frequency of the original sound, and selecting and recording the lower side band.

9. In a systemfor recording a band of audio frequency waves which comprises means for modulating an audio frequency carrier wave in accordance with said waves, means for selecting the lower side band from the resulting modulated wave, and means for recording said lower side band. i

10. A'system for recording sound which comprises means'for modulating. an audlo frequency carrier wave in accordance with speech, means for selecting from the modulated wave the lower side band comprising audible but unintelligible inverse frequency speech waves and means for recording said inverse frequency speech waves.

11. In a system for the transmission of intelligence by a band of audio frequency waves of a given frequency range com-prising means forinverted frequencies of the components thereof to produce a band of unintelligible audio frequency waves of frequency range substantially equal to that of the band of Waves to be transmitted, means for making a record of said unintelligible waves, means reproducing said recorded waves and means for reinverting the frequencies of the frequency components of the recorded energy to reproduce the first side band of audio frequency waves.

In witness whereof, I hereunto subscribe my name this 24th day of October A. D.,

JOHN MILLS. 

